1. Field of the Invention
This invention relates to a magnetic memory element based on the magnetoresistance effect and a magnetic memory apparatus including the magnetic memory elements.
2. Background Art
Recently, there has been a growing demand for information processing devices that meet various needs as an underpinning and engine for the extensively and highly advanced information society. In particular, hard disk drives and magnetoresistive random access memory (MRAM) are memory devices based on the magnetic moment of ferromagnets. Such spin-electronics devices using the spin degree of freedom of electrons are characterized in being suitable to increasing integration by downsizing cells, operable at high speed, and nonvolatile. Hence their use will further expand in memory apparatuses and other applications.
In one method for controlling the magnetization direction of small magnetic bodies in spin-electronics devices, the current-induced spin transfer phenomenon is used. The “spin transfer” refers to the transfer of angular momentum from the spin of conduction electrons to the localized magnetic moment of the magnetic bodies. In contrast to the scheme based on magnetic field application, the spin transfer scheme is characterized in that the write current can be reduced with the downsizing of cells.
The magnetic memory element adapted to spin transfer writing is illustratively shaped like a pillar. More specifically, it includes a lamination film composed of a magnetization-pinned magnetic layer (hereinafter also referred to as “pinned layer”), an intermediate layer, and a magnetization-free magnetic layer (hereinafter also referred to as “memory layer”). The lamination film is patterned with dots, each being tens to hundreds of nanometers square. Both the pinned layer and the memory layer are magnetized in the direction within the film plane. By passing a current through this lamination film in the direction perpendicular to the film plane, the magnetization direction can be controlled (write) and detected (read), and the element can be used as a memory element.
Here, one memory element can include a plurality of memory layers, and each memory layer can independently serve for recording, storing, and reading of data.
Several publications (U.S. Pat. No. 6,304,477, JP-A 2002-334585 (Kokai), and JP-A 2005-259206 (Kokai)) disclose incorporating such memory elements into a circuit to form a CAM (content addressable memory). The CAM, also called “associative memory”, is a functional memory capable of comparing a first data (bit string) with a second data (bit string) and rapidly outputting a determination as to whether they are matched. There is a great demand for CAMs in routers and caches. Besides CAMs, a magnetic memory capable of multivalued operation has been disclosed (JP-A 2006-203064 (Kokai)).